Huygens (crater)
Encyclopedia
Huygens is an impact crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

 on Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 named in honour of the Dutch astronomer, mathematician and physicist Christiaan Huygens.

The crater is approximately 456 km in diameter and can be found at 304.4°W 14.0°S.
It is located in the Iapygia quadrangle
Iapygia quadrangle
The Iapygia quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey Astrogeology Research Program. The Iapygia quadrangle is also referred to as MC-21 ....

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Scientists were delighted to see branched channels in pictures taken with spacecraft that were sent in orbit around Mars. The existence of these channels is strong evidence that much water once flowed on the surface of the planet. Simple organisms may have once lived where water once was. An excellent group of these channels are shown in the picture below from the rim of Huygens taken with THEMIS
Themis
Themis is an ancient Greek Titaness. She is described as "of good counsel", and is the embodiment of divine order, law, and custom. Themis means "divine law" rather than human ordinance, literally "that which is put in place", from the verb τίθημι, títhēmi, "to put"...

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Carbonates (calcium or iron carbonates) were discovered in a crater on the rim of Huygens. The impact on the rim exposed material that had been dug up from the impact that created Huygens. These minerals represent evidence that Mars once was had a thicker carbon dioxide atmosphere with abundant moisture. These kind of carbonates only form when there is a lot of water. They were found with the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument on the Mars Reconnaissance Orbiter
Mars Reconnaissance Orbiter
Mars Reconnaissance Orbiter is a NASA multipurpose spacecraft designed to conduct reconnaissance and Exploration of Mars from orbit...

. Earlier, the instrument had detected clay minerals. The carbonates were found near the clay minerals. Both of these minerals form in wet environments. It is supposed that billions of years ago Mars was much warmer and wetter. At that time, carbonates would have formed from water and the carbon dioxide-rich amosphere. Later the deposits of carbonate would have been buried. The double impact has now exposed the minerals. Earth has vast carbonate deposits in the form of limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

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